Method of increasing the yield of accelerated particles in a betatron or synchrotron



March'IZ, 1968 .|PEK ET AL 3,373,325

METHOD OF INCREASING THE YIELD OF ACCELERATED PARTICLES IN A BETATRON OR SYNCHROTRON Filed Oct. 24, 1965 PAR TIC L 14 ACCELERATOR 1 .l I E QPAR r/cus -14 Acca mrok INVENTORS.

Lam'seav z'pe/i BY Jan Dvorak United States Patent 3,373,325 METHOD OF INCREASING THE YIELD OF AC- CELERATED PARTICLES IN A BETATRON 0R SYNQHROTRON Ladislav Sipek and Jan Dvofak, Prague, Czechoslovakia, assignors to Ceskoslovenska akademie vd, Prague, Czechoslovakia Filed Oct. 24, 1963, Ser. No. 318,632 Claims priority, application Czechoslovakia, Nov. 2, 1962, 6,206/ 62 4 Claims. (Cl. 317-262) The invention is directed to a circuit arrangement for increasing the yield of accelerated particles in a betatron or synchrotron, using a magnet fed from a source of alternating voltage, preferably from conventional supply mains.

Accelerators of the betatron or synchrotron type operate cyclically. The particles are accelerated in repeated cycles as the applied magnetic field increases from a value close to zero. It is obvious that the amount of acceleration imparted to the particles in a unit of time, or in other words the intensity of radiation attained by the accelerator used as the radiation source depends, among other factors, on the number of accelerating cycles in a unit of time.

It has been proposed to increase the yield of accelerated particles by increasing the number of accelerating cycles in a unit of time by increasing the frequency of the supply voltage which energizes the magnet. However, this method is attended with the drawback of a rather large increase in the iron loss in the magnet core, with the loss increasing the frequency of the supply voltage. Another drawback of this method resides in the fact that the composite arrangement requires a special, powerful source of alternating voltage at the higher frequency. In adition, only one half-cycle of the exciting magnet current is effective.

It has also been suggested that the second half-cycle of the exciting magnet current be utilized. However, this is attended with the drawback that the direction of movement of the accelerated particles is reversed in each halfcycle, since magnetic fields of an opposite polarity are generated. In connection with accelerators used as a source of gamma radiation, two modifications of conventional accelerator organizations have been employed, but neither of them can be said to be sufiiciently satisfactory. In accordance with the first modification the radiation is emitted from .a single target electrode, but its direction changes in accordance with the direction of the magnetic field in each half-cycle of the exciting current. In accordance with the second modification the radiation is always emitted in the same direction in both half-cycles of the exciting magnet current, but in this case it has to be emitted from two target electrodes spaced from each other by the diameter of the balance path of the accelerated particles, for example in a stereobetatron.

Broadly speaking, an object of the invention is to eliminate the said drawbacks.

More specifically, an object of the invention is to make use of both half-cycles of the exciting magnet voltage without increasing the source frequency. The windings and sources are arranged in such a manner that both halfcycles of the exciting alternating potential, for example from the mains, always generate an exciting field or" the same direction.

In this manner, as will be shown below in more detail, it is even possible to achieve a little more than double the yield of accelerated particles, with operational costs remaining approximately the same, and production costs being only slightly higher.

The more specific object of the invention is to provide a method of, and a circuit arrangement for, increasing the yield of accelerated particles in a betatron or synchrotron by increasing the number of accelerating cycles in a unit of time. To this end, the system of magnet windings of the accelerator is switched over by known means in the half-cycles of the alternating supply voltage in such a manner that the current which excites the magnetic field is pulsating and has always the same polarity, and the moment of connecting the windings to the source is chosen in the vicinity of the moment of maximum voltage value and the moment of disconnection is near the point where the current passes through zero value.

An arrangement for carrying out the method according to the invention consists of a system of magnet windings formed by at least one coil to the terminals of which is connected the output of a system of switches which is synchronously controlled by the frequency of the alternating supply voltage and fed from the source of alternating voltage.

In particular, a specific illustrative particle accelerating arrangement made in accordance with the principles of the present invention is adapted to supply interrupted, unidirectional current pulses to an accelerating magnet winding. The arrangement comprises a bridge configuration embodied by four controlled rectifiers, such as thyratrons, with the winding and a source of alternating potential being connected to diagonally opposite bridge node pairs.

A synchronous control device is employed to render opposite bridge rectifier pairs conductive during a portion of the interval when the voltage source is translating between peak potentials of an opposite polarity. By reason of the rectifier characteristics, interrupted, unidirectional current pulses are created in the winding, hence generating a proper field for particle acceleration.

It is thus a feature of the present invention that a particle accelerating organization include a bridge configuration comprising four controlled rectifiers, a field generatingaccelerator winding, a source of line voltage connected to a first pair of opposite bridge nodes and the winding connected to the remaining bridge nodes, and a synchronous control device for selectively and alternately enabling the two opposite bridge rectifier pairs.

The invention will be best understood from the following specification to be read in conjunction with the accompanying drawing. In the drawing:

FIG. 1 shows the current and voltage timing curves for an alternating voltage source and a magnet winding system;

FIG. -1 illustrates an example of circuit arrangement in accordance with the invention with a switch comprising a controlled rectifier of the Graetstype; and

FIG. 3 illustrates an example of circuit arrangement in accordance with the invention with a switch comprising a controlled rectifier with a double winding. i

Referring now to FIG. 1, it can be seen from curve 1 that if the-magnetic field of the accelerator is to be pulsating in one direction and to be increasing from a value close to zero, the exciting current must also have an identical shape developed in time. Curve 2 which is drawn partly in broken line represents the curve of the alternating current supply voltage and curve 3 which is drawn in full line represents the voltage curve on the terminals of the electromagnet winding. The time section 4 represents the time interval during which the current 1 in the electromagnet winding is interrupted. For simplicitys sake, the curves are given an idealized shape; this is sufiicient for understanding the invention. Due to the fact that the magnet winding system of the accelerator represents a purely inductive impedance, the phase of the voltage curve leads the current curve by and at the moment of zero current it has to be commutated, that is to say, its direction has to be changed, as can be seen from curves 1 and 2. The curve 2 of the alternating supply voltage is illustrated by the partly broken line. Interruption of the current which is essential for the magnetic field in the accelerator to be zero at certain moments, or at least close to zero, lasts and for a time period whose length is marked 4. This length is adjustable by controlling the moments when the winding energizing current path is completed and interrupted near the peak values of the supply voltage. The current curve 1 is automatically symmetrical with regard to the moment at which the voltage 2 passes through zero, for any moment at which the current circuit into the magnet winding is switched-on. If current interruption in the magnet winding is secured within the time interval 4, the magnet field is also practically zero, and after applying the voltage it can begin to increase from zero value, and the accelerating cycles will proceed normally.

Due to the fact that at the moment of voltage commutation on the magnet terminals no current should flow through the magnet, safe current interruption should be secured. It is therefore necessary to use synchronously controlled current switches to ensure that at the desired moments during the time interval 4 the magnet winding current, and thus also its magnetic field, would be zero.

FIG. 2 shows an illustrative circuit arangement for embodying the above-described functional concepts. In this figure, 6 is the winding of the accelerator magnet, and 16 are magnet winding terminals, 7, 8, 9 and 10 are thyratrons or ignitrons arranged in a controlled rectifier bridge, 11 is a capacitor which adjusts the power factor, 12 and 13 are mains terminals, and 14 comprises a known device for controlling and regulating the moment of ignition of the ignitrons 7, 8, 9 and 10. The control device 14, which may embody any of the plural well known cornmutating and synchronous structures operates the ignitrons in such a manner that two of them in opposite bridge branches are rendered conductive at the same time, that is the pair 7, 9 or 8, 10. As soon as the ignitrons of one pair are ignited by the device 14 at a supply voltage peak, the current 1 begins to flow through the magnet winding 6. After completion of one half-cycle, the current drops to a value at which the arc in the ignitron is extinguished, and this interrupts the current in the magnet winding 6. The device 14 next causes ignition of the second pair of the ignitrons which are supplied with sufficient voltage for creation of an arc. The current 1 in magnet winding 6 begins to again increase in the same direction as in the preceding half cycle. Since the ignitrons 7, 9 and 8, 10 are ignited in the vicinity of the maximum of the supply voltage 2, current curves 1 as in FIG. 1 are automatically created, as is necessary for correct operation of the accelerator.

Another circuit arrangement with more exacting demands on the construction of the magnet winding is illustrated in FIG. 3. The reference numbers in this figure correspond to those used in FIG. 2. The magnet winding has to be divided into two similar, oppositely poled parts 6 and 6". In this case two controlled ignitrons 7 and 8 operated in succession are sufficient to energize the winding parts 6 and 6 in a manner which creates the desired unidirectional magnetic field. But in the reverse direction they carry double source voltage.

It should be obvious from the above description of the illustrative commutating circuit arangements that instead of ignitrons, other elements of a similar function may also be used as controlled switches, for example thyratrons or controlled semiconductor diodes. It should also be understood by those expert in the art that the same effect may be obtained by controlled contacts or by a rotary commutator.

It should also be understood that the invention may also be used without difiiculty with some other types of accelerators with a combined alternating and direct current field, such as for example with FFAB betatrons.

What we claim is:

1. In combination in a particle accelerating organization, particle accelerating means including winding means for generating a unidirectional magnetic field, and energizing means connected to said winding means for energizing said winding means with unidirectional current pulses spaced from each other by a determined time interval, said energizing means comprising a voltage source for supplying an alternating potential having a determined frequency which is half the repetition rate of said current pulses, said alternating potential being characterized by maximum values in a first and second reference polarity, first controlled switching means for completing a first conduction path between said voltage source and said winding means, second controlled switching means for completing a second conductive path between said voltage source and said winding means, and control means for energizing said first and second switching means between the times when said voltage source is supplying a peak voltage in said first and second, and said second and first reference polarities, said peak voltage determining the duration of said determined time interval.

2. A combination as claimed in claim 1, wherein said first and second controlled switching means respectively comprise controlled rectifiers included in opposite branches of a bridge configuration, and wherein said potential source and said winding means are connected to opposite bridge node pairs.

3. A combination as claimed in claim 1, wherein said winding means comprises first and second serially interconnected winding parts having a junction point therebetween connected to said voltage source, and wherein said first and second controlled switching means comprise first and second controlled rectifiers respectively joining the uncommon ends of said first and second winding parts with said voltage source.

4. In combination in a particle accelerating arrangement, four controlled rectifiers connected in a bridge configuration, means for supplying an alternating potential having a determined frequency and exhibiting peak values in each polarity connected to a first pair of opposite bridge nodes, particle accelerating means including winding means for generating a unipolar magnetic field, said winding means being connected across a second pair of opposite bridge nodes, and commutating means for energizing alternate pairs of opposite bridge rectifiers when the voltage source respectively supplies its two, opposite polarity peak voltages whereby said bridge configuration provides to said winding means unidirectional current pulses spaced from each other by a determined time interval having a duration determined by said peak voltages, said current pulses having a repetition rate which is twice the frequency of said alternating potential.

References Cited UNITED STATES PATENTS 2,538,718 1/1951 Widerol 328-237 2,654,838 10/1953 Widerol 328-237 2,970,247 1/1961 Hill 317-123 1,115,456 10/1914 Thomas 315-253 LEE T. HIX, Primary Examiner.

SAMUEL BERNSTE-IN, Examiner. 

1. IN COMBINATION IN A PARTICLE ACCELERATING ORGANIZATION, PARTICLE ACCELERATING MEANS INCLUDING WINDING MEANS FOR GENERATING A UNIDIRECTIONAL MAGNETIC FIELD, AND ENERGIZING MEANS CONNECTED TO SAID WINDING MEANS FOR ENERGIZING SAID WINDING MEANS WITH UNIDIRECTIONAL CURRENT PULSES SPACED FROM EACH OTHER BY A DETERMINED TIME INTERVAL, SAID ENERGIZING MEANS COMPRISING A VOLTAGE SOURCE FOR SUPPLYING AN ALTERNATING POTENTIAL HAVING A DETERMINED FREQUENCY WHICH IS HALF THE REPETITION RATE OF SAID CURRENT PULSES, SAID ALTERNATING POTENTIAL BEING CHARACTERIZED BY MAXIMUM VALUES IN A FIRST AND SECOND REFERENCE POLARITY, FIRST CONTROLLED SWITCHING MEANS FOR COMPLETING A FIRST CONDUCTION PATH BETWEEN SAID VOLTAGE SOURCE AND SAID WINDING MEANS, SECOND CONTROLLED SWITCHING MEANS FOR COMPLETING A SECOND CONDUCTIVE PATH BETWEEN SAID VOLTAGE SOURCE AND SAID WINDING MEANS, AND CONTROL MEANS FOR ENERGIZING SAID FIRST AND SECOND SWITCHING MEANS BETWEEN THE TIMES WHEN SAID VOLTAGE SOURCE IS SUPPLYING A PEAK VOLTAGE IN SAID FIRST AND SECOND, AND SAID SECOND AND FIRST REFERENCE POLARITIES, SAID PEAK VOLTAGE DETERMINING THE DURATION OF SAID DETERMINED TIME INTERVAL. 